Models for Calculating the Effect of Environment on Fatigue Life (F_en) for Complex Waveforms and/or Non-Isothermal Conditions

Environmentally assisted fatigue of nuclear plant materials in the Pressurised Water Reactor (PWR) coolant environment is a phenomenon that has been extensively studied over the past 30 years. Methods for accounting for the PWR environment in an ASME III fatigue assessment are presented in NUREG/CR-6909. The deleterious effect of environment is described through a F_en factor dependent upon strain rate, temperature and the dissolved oxygen content of the water. The formulae which describe the F_en are based upon correlations observed in test data predominantly from tests conducted with constant temperature and strain rate (triangular or sawtooth loading). Actual loading histories encountered during service are far more complex, with both strain rate and temperature, and therefore F_en, varying through the cycle. NUREG/CR-6909 recommends a modified rate approach method for accounting for this, whereby the load cycle is broken up into a number of strain increments and then integrated to give the F_en for the cycle.

There is a substantial and growing body of data for conditions in which the strain rate or temperature or both changes. The NUREG/CR-6909 modified rate approach does not generally perform well in describing the deleterious effect of environment in these complex conditions. In particular, the modified rate approach does not predict a difference in life when the temperature is varied in-phase or out-of-phase with the strain waveform, or when the slow portion of the strain rate is moved from the top to the bottom of the waveform.

This paper presents new data from strain-controlled fatigue endurance testing of two casts of 304L with complex waveforms and temperature cycling. The paper then presents and compares a number of models for integrating the F_en through the cycle, including methods which weight F_en increments depending on position in the strain cycle.

It is concluded that greater weighting on the environmental effect in the top of the cycle is necessary to describe the differences in life observed. This is further validated by a review of test data in the wider literature. An improved method is presented to account for the effects of the PWR environment on fatigue lives of austenitic stainless steel materials, which has similarities to the “Weighted K Rate” method previously presented by Rolls-Royce, PVP2016-63497, for environmentally assisted fatigue crack growth.